The objectives of this project are the continuation of work already begun in this laboratory on several aspects of human prolactin. These include: 1. Further characterization of the hormone in blood by in vitro mouse breast bioassay, radioimmunoassay, and radioreceptor assay. 2. Continuing exploration of the physiological and pharmacological factors affecting the regulation of prolactin secretion; particular attention will be paid to factors which affect the recently discovered sleep-induced rise; also studied will be the effects of estrogens and progestins and the mechanisms governing prolactin release in sexual intercourse. 3. The relationship of prolactin secretion to that of TSH will be studied, as well as to that of vasopressin and growth hormone. 4. The role of prolactin in breast cancer will be further investigated, with emphasis particularly on the attempt to treat metastatic human breast cancer with pharmacological suppression of prolactin by L-dopa and other agents. 5. Also studied will be abnormalities of prolactin secretion in pituitary tumors, both by radioimmunoassay of plasma and by specific hormonal staining of pituitary tumors removed at surgery by immunoenzyme techniques.